Modern dental procedures often involve the fabrication of restorations such as crowns, implants, fixed partial dentures, and veneers. Ceramics are often used in such restorations because their optical properties are such that skillfully produced ceramic restorations can closely match the shape, texture, color and translucency of natural teeth.
Producing such realism involves a considerable degree of skill. Moreover, it requires that information regarding the color and other appearance characteristics of a patient's teeth be accurately determined and unambiguously conveyed to those who will be fabricating the restoration. While molds and other techniques can be used to record and transfer information regarding tooth shape and other geometric characteristics, techniques for determining and conveying color and other appearance characteristics are more problematic.
The most widely used techniques for determining and communicating tooth color information have changed little in the past seventy years. Typically, the process (referred to as “shade matching”) involves visually matching a patient's tooth to one of a number of reference shade samples (shade tabs) within one or more sets of standardized shade guides. The person performing the match, often a dentist, records the identification of the matching shade tab and conveys that information to the dental laboratory where the restoration will be fabricated. The laboratory then uses its own set of the same shade guides to perform visual color evaluations of the restoration throughout the fabrication process.
The conventional visual shade matching process has a number of problems. The initial matching procedure is often long, difficult, and tedious. It is not unusual for the process to take twenty minutes or longer. In most cases, there will be no shade tab that perfectly matches the patient's teeth. Deciding which tab matches most closely (i.e., which mismatches the least) is often difficult. Frequently, the dentist may determine that the patient's teeth are particularly difficult to match. If this is the case, the patient may need to personally visit the orthodontics laboratory that will be fabricating the restoration. There, trained laboratory personnel can perform the color match. In many cases, the patient may then need to return to the dentist and laboratory two, three, or even more times as the color of the prosthetic is fine tuned by sequential additions of ceramics or other colored materials.
Despite the time and effort expended, this conventional visual color matching procedure fails (i.e., the prosthetic is rejected for color by the dentist and/or the patient) in about 10% of cases. Given the difficulty of the task, this rate of failure is not at all surprising. Visual color evaluation of relatively small color differences is always difficult, and the conditions under which dental color evaluations must be made are likely to give rise to a number of complicating psychophysical effects such as local chromatic adaptation, local brightness adaptation, and lateral-brightness adaptation. Moreover, shade tabs provide at best a metameric (i.e., non-spectral) match to real teeth; thus the matching is illuminant sensitive and subject to variability due to normal variations in human color vision (e.g., observer metamerism).
The difficulties associated with dental color matching have led to the development of a number of systems that attempt to replace visual assessments with instrumentation-based assessments using various types of spectrophotometric and calorimetric instruments. Although the idea of basing shade matching on objective measurements rather than on subjective visual color assessments seems appealing, such measurements are extremely difficult to perform in practice. As a result, reports from dentists and dental laboratory personal indicate that the level of performance of currently available instrument-based shade matching systems is not entirely acceptable. Uncertainties resulting from available instrument-based systems generally require that traditional visual assessments must still be performed for verification. Thus, much of the value of such systems is largely negated.
The failures and limitations of currently available shade-matching systems, both instrument-based and visual-based, can best be understood by examining the difficulties involved in matching the appearance of human teeth. First, tooth color is a complex interaction of reflection, transmission, refraction, fluorescence, and scattering by a variety of organic and inorganic components. It is influenced by variations in tooth pulp volume, dentin condition, enamel composition, and other variations in the composition, structure, and thickness of the dental tissues. One result of this complexity is that color appearance and color measurement are greatly influenced by lighting geometry, surrounding colors, and other environmental factors.
A further complication is that color is generally not uniform within a single tooth. Color non-uniformities may result from spatial variations in composition, structure, thickness, internal and external stains, surface texture, fissures, cracks, and degree of wetness. As a result, measurements based on relatively large areas produce averaged values that may not be representative of a tooth's dominant color. In addition, natural color variations and non-uniformities make it unlikely that a given tooth can be matched exactly by any single shade tab. This means that a method for conveying the distribution of color within a tooth, not just its average color, is required. Tooth color also is seldom uniform from tooth to tooth. Therefore, the ideal color of a restoration may not be an exact match to that of an adjacent tooth or to any other single tooth in a patient's mouth. Dentists use the word “harmony” to describe how a restoration should appear to blend with the various colors of a patient's teeth. The ideal color of a restoration may, for example, be somewhere between that of several nearby teeth, or it may be closer to the color of a similar tooth elsewhere in the mouth.
A further difficulty is that successful color communication requires that tooth color can be measured and specified according to a set of absolute reference color standards, such as numeric colorimetric values or reference shade tab identifiers. It is particularly important that the luminance factor is determined and conveyed accurately; yet luminance factor generally is the most difficult aspect of color to measure. Furthermore, error tolerances for all aspects of tooth color are extremely small. In the mouth, a reconstruction such as a single crown is immediately adjacent to natural teeth. This proximity makes even small color errors very apparent. Moreover, people generally are particular about the appearance of their teeth. Understandably, they are quite intolerant of restorations that appear inappropriate in color.
Lighting is an additional source of difficulty in performing dental color measurements. The type of lighting, the lighting geometry, and other factors must be appropriate for measurement purposes. In particular, specular reflections from the tooth surface must be avoided. At the same time, however, the measurement conditions must be consistent with those under which the results ultimately will be judged. These two needs are often in conflict; optimum conditions for making dental measurements generally are quite different from those of the real world.
Additionally, visual color assessments and objective measurements must be made in inherently difficult environment, i.e., the mouth of a live patient. Factors such as hygiene, aesthetics, and patient comfort are important and must be considered in the design of the assessment or measurement techniques. Speed is also a concern. If the patient's mouth is open, the teeth begin to dry in a relatively short period of time. This drying changes the relative refractive index of the surface, which lightens and desaturates the apparent color of the teeth. Instrument measurements or visual matches made under such conditions will likely lead to poorly matched prosthetics.
Color assessments, specifications and communication are further complicated by a lack of accurate color calibration within the dental industry. For example, studies have shown that there can be considerable variation even among supposedly identical sets of shade tabs from the same manufacturer. These variations make color communication based on such tabs ambiguous. For example the matching shade tab selected by a dentist may differ from the actual tab that will be used for reference at the laboratory fabricating the prosthetic, even though both tabs have the same identification and are assumed to be identical. As a result, a prosthetic built to match the color of the laboratory's shade tab will not match the color intended by the dentist. It would be valuable, then, for both the dentist and the dental laboratory to have a reliable and unambiguous means for specifying color. In addition, it would be valuable for the dentist and/or dental laboratory to have a means for verifying that a restoration meets a prescribed shade specification. At a dental laboratory, it would also be valuable to have a verification process incorporated in the fabrication process to provide shade guidance at intermediary stages of that process. Color adjustments then could be incorporated in subsequent fabrication stages.
Although a number of shade-matching systems have been described in the prior art, none fully addresses all the issues addressed above. For example, in a series of patents, including U.S. Pat. Nos. 6,358,047; 6,305,933; 6,206,691; 6,132,210; and 5,961,324 (all to Lehmann et al.), disclose a tooth shade analyzer system in which the preferred embodiment is based on the use of an intra-oral camera providing red, green, and blue (RGB) color values that are subsequently normalized and then used to derive hue, saturation, and intensity (HSI) values using a single set of RGB-to-HSI conversion equations. The derived HSI values are then compared to those derived from corresponding RGB measurements taken of a collection of shade tabs. Similarly, in U.S. Pat. Nos. 6,190,170 and 6,328,567 (both to Morris et al.) a system that uses two or more references to normalize RGB image values from one or more digital cameras is disclosed. Again, teeth and shade tabs are compared according to their RGB values or to HSI or other values derived from RGB values using a single set of conversion equations. Similarly, U.S. Pat. No. 6,384,917 (Fradkin) discloses a system that uses beam splitters and other optical components to obtain RGB image values. Once again, teeth and shade tabs are compared according to their RGB values or to HSI or other values derived from RGB values using a single set of conversion equations. U.S. Patent Application Publication No. 2002/0021439 A1 (Priestley et al.) also discloses a color matching system in which colors are analyzed in terms of RGB values. The underlying assumption in all these descriptions is that the color of a tooth (i.e., its visual color appearance) can be matched by a shade tab having the same RGB values (or HSI or other values derived from those RGB values using a single set of conversion equations). However, that assumption is not generally true. The spectral reflectances of shade tabs differ from those of natural teeth; thus, visual matches between teeth and tabs are metameric, rather than spectral, matches. Furthermore, the spectral sensitivities of current digital cameras, including conventional and intra-oral cameras, are not equivalent to a set of visual color matching functions. As a result, matches determined from RGB measurements, or from HSI or other values derived by applying any given single set of conversion equations to measured RGB values, generally do not result in accurate visual matches. It is quite possible, for example, that a tooth and a shade tab may have identical RGB values, and thus identical derived HSI values; but still not match visually. It is also quite possible that a tooth and a shade tab may have different RGB values, and thus different derived HSI values; yet match visually. Such occurrences are a consequence of the basic nature of metameric matching.
U.S. Pat. No. 6,007,332 (O'Brien) discloses a tooth color matching system based on producing a photograph image of a tooth together with a visually selected color standard, such as a dental shade tab, and analyzing the photographic image using a calorimetric or spectrophotometric device. Using colorimetric or spectrophotometric devices in this manner does not address the fundamental problems associated with the metamerism of natural teeth and shade tabs because the devices are used to analyze the color of the resulting photograph, not the color of the original tooth and shade tab. The color comparison therefore will be subject to metamerism problems resulting from the fact that, like digital cameras, photographic media have RGB spectral sensitivities that are not equivalent to a set of visual color matching functions. Thus, a tooth and shade tab that match perfectly in the photographic image (visually, calorimetrically, and spectrally) still may not match visually in real life.
Other shade-measuring systems attempt to avoid problems related to metameric matching by using spectrophotometers or calorimeters for direct shade measuring. However, the geometry and other characteristics of the lighting used on such systems described in the prior art generally do not correspond to the lighting conditions under which teeth normally would be viewed. Spectrophotometric or calorimetric measurements made under non-representative lighting conditions may produce non-representative color values that result in unsatisfactory visual matches under normal viewing conditions. Moreover such systems do not provide images of the full mouth, or even of adjacent teeth. Thus, they do not provide information required to ensure a shade match that is harmonious in color with the surrounding teeth and mouth structure, nor do they convey other important information related to tooth appearance such as texture and gloss.
Some camera-based solutions described in patent literature also rely on lighting that is not representative of the lighting conditions under which teeth normally are viewed. For example, U.S. Patent Application Publication No. 2002/0021439 (Priestley et al.) discloses a system in which cross-polarization is used to reduce glare from the tooth front surface. However, shade matches achieved under such lighting conditions may not necessarily match under more normal ambient light conditions. U.S. Pat. No. 5,759,030 (Jung et al.) discloses a method for determining optical characteristics of teeth in which light is provided by a central source fiber optic and detected by an array of perimeter receiver optics. Again, shade matches and other tooth characteristics measured under such lighting conditions may not necessarily correspond to those observed under normal conditions.
Commonly-assigned copending U.S. patent application Ser. No. 10/460,693 (Giorgianni et al.) addresses methods and requirements for obtaining an accurate colorimetric profile from data measured from the tooth. The matching of natural teeth with shade tabs and prosthetics is fundamentally metameric, not spectral. In addition, natural teeth and some ceramic materials exhibit fluorescence and other behaviors influenced by lighting characteristics. Therefore, the spectral energy distribution properties of the light source used for shade matching must be representative of sources under which the quality of the prosthetic ultimately will be judged. For measurement purposes, the lighting must provide an area of illumination that is substantially uniform in the plane of the target tooth and of an intra-oral reference. The illumination must also be sufficiently uniform in depth so that the front-to-back positioning of the target tooth is not overly critical. For measurement purposes, the lighting must minimize or eliminate specular reflections from the measurement areas of the target tooth and intra-oral reference. For measurement purposes, the lighting apparatus must provide repeatable illumination angle, intensity, and chromaticity from one exposure to the next. (However, the use of an appropriate photographic reference and the exposure-compensation methods of this invention can somewhat reduce the requirements for such repeatability.) The light available prior to exposure must be sufficient to allow proper positioning of the patient and focusing of the camera. Ideally, there should be a provision for previewing the lighting prior to camera exposure.
Patient comfort is a key consideration for illumination components. The lighting system should not cause the patient discomfort, such as by producing an undue amount of heat. The lighting pattern should be restricted so that little or no direct light reaches the patient's eyes. Bulky and cumbersome apparatus requiring continual checks or adjustments would not be favorable; ideally, illumination should require little or no adjustment. Test references that provide benchmark calibration should be manageable in size and, ideally, should not require the assistance of a technician or patient to hold the reference in place while a measurement is obtained.
As described in the commonly-assigned U.S. patent application Ser. No. 10/460,693 by Giorgianni et al. application cited above, an illumination angle of substantially 30 degrees has been found to be most favorable for shade matching. Uniform illumination provided at this optimal angle helps to minimize specular reflection that can have an adverse affect on color shading data. It has also been observed that the use of a fixed angle of illumination is important; otherwise, even slight angular changes in incident light can affect the colorimetric profile obtained.
In addition to requirements for suitable, fixed illumination angle, for positioning of a reference target, and for overall patient comfort, there are other practical considerations, including cost, size, flexibility, adaptability for use without requiring training, and ease of use in the dental examination environment. Thus, it can be seen that there is a need for an apparatus that enables color measurements for color shade matching and meets demanding requirements for accuracy, suitability, and ease of use.
In a broader context, there are other applications for which control of various illumination characteristics such as incident angle, color temperature, and brightness can significantly impact the usefulness of the images obtained. Applications of special interest can be found in dental and medical imaging, machine vision applications, and surface inspection applications, for example. Thus, in addition to meeting specific needs for improvement of dental color measurement as outlined above, there would also be benefits to imaging illumination solutions that serve a wide range of uses.